It is well known that SiO.sub.2 -based single mode optical fibers having zero dispersion at a wavelength of 1.3 .mu.m have a remarkably large chromatic dispersion of 15 ps/km.multidot.nm or more at a wavelength band of 1.5 .mu.m, and are thus not suitable for high bit rate and long-haul optical transmissions at a wavelength of 1.5 .mu.m.
As a means of overcoming this limitation, dispersion shifted optical fiber, in which a zero dispersion wavelength is shifted to 1.5 .mu.m band having a dual-shaped core profile shown in FIG. 5, has been proposed as an optical fiber which enables high bit rate and long-haul optical transmissions.